ALEPPO: The Syrian army launched a massive assault on rebels in Aleppo on Saturday amid growing world concern about the risks of reprisals against the civilian population of the country's second city.

Troops backed by tanks and helicopter gunships, which had been massing for the past two days, moved on southwestern districts of the commercial hub, where rebel fighters concentrated their forces when they seized much of the city on July 20.

At least 90 people were killed across Syria on Saturday , including 29 in Aleppo. In the past week, at least 100 people have been killed every day, including on Friday, when at least 148 people were killed, according to the Syrian Observatory for Human Rights.

Artillery pounded Salaheddin and other rebel neighbourhoods from 1030 IST as ground troops made their advance . Trapped civilians crowded into basements, seeking refuge from the intense bombardment.

"The fiercest clashes of the uprising are taking place in several neighbourhoods of the city," the head of the watchdog, Rami Abdel Rahman, said.

At least 10 soldiers and three rebels were killed in fierce fighting after the launch of the assault, the Observatory said.

"The regime's forces tried to storm the headquarters of Salaheddin but thank God, the heroes of the (rebel) Free Syrian Army repulsed the attack," FSA Col Abdel Jabbar l-Oqaidi said.

"There are 100 tanks massed on the outskirts of the district. The battle will be hard because there is no balance of forces," he added.

More than 20,000 people have been killed in Syria since the start of the anti-regime revolt in March 2011, the watchdog said on Friday. The opposition fighters had been holding their fire in readiness for the threatened assault by the army.

But their small arms and rocket-propelled grenades were little match for Assad's heavy armour.